Some hydraulic excavators and other work machines are designed such that potential energy of a work equipment is recovered to be used for hydraulic pressure source or assisting operation of actuators.
Taking the up-and-down operation of the work equipment by boom cylinders as an example, when a boom that has been raised is moved down, the oil at the head side of the boom cylinders is pushed out under a high pressure by the potential energy of the boom. Should the oil of which the pressure has become high be transformed to thermal energy by means of throttling in the hydraulic circuit or returned to the tank without being utilized, it would be wasteful. Therefore, various energy recovery systems have been proposed, including one shown in FIG. 6, and others similar thereto (e. g. see PTL 1 and PTL 2). With the energy recovery system shown in FIG. 6, oil of which the pressure has become high at the head side of a boom cylinder 1 is fed to an accumulator 5 through a solenoid-operated control valve 2, a poppet valve 3, and a check valve 4 so that the pressure is stored in the accumulator 5, and when an actuator, such as a boom cylinder 1, is moved, the oil stored in the accumulator 5 is released through a pilot-operated control valve 6 and a check valve 7 to a discharge line that serves to feed hydraulic oil from a main pump 8 to a main control valve 9, thereby enabling effective use of the potential energy of the boom.